Communication systems frequently have multiple communication links operating in parallel to increase the speed of transmission, and/or to simplify the construction of an individual link. By way of example, U.S. Pat. No. 6,873,630 incorporated herein by reference, issued Mar. 29, 2005 to Muller et al., discloses an Ethernet transmission system architecture that distributes individual Ethernet frames across a plurality of logical channels running in parallel at much lower data rates. Each channel may be conveyed by a separate conductor, or the channels may be carried through a shared medium using a form of multiplexing. The conductors may be electric or fiberoptic cables.
An emerging IEEE 802.3ba standard “40 Gb/s and 100 Gb/s Ethernet”, which is incorporated herein by reference, involves a data transmission at a very high rate using parallel operation of many individual lower data rate links. The parallel operation of the individual links provides the specified data rate. The IEEE 802.3ba standard has many variants. Some of the IEEE 802.3ba variants specify electrical cabling equipment to be used to support the lower data rate links, while others specify fiber optics equipment to be used to support the lower data rate links.
The emerging IEEE 802.3ba standard defines many requirements, as well as tests to confirm that the requirements are met. Some of the requirements are specifications on the transmitters, such as output voltage or power levels. In the case of fiber optics, the transmitters must operate within a certain wavelength range. Because the transmitters are often placed in close proximity to each other and to receivers, there are tests needed to confirm that they do not suffer from excessive cross-talk from each other's simultaneous operation. Other requirements relate to the receivers, which must satisfy sensitivity requirements and overload performance requirements. Still other requirements relate to the system functioning in the presence of varying relative delay of transmission among the links. This varying relative delay is called herein “skew”.
Referring to FIG. 1a, a prior-art communication link 104 comprises a transmitter (TX) 10, a transmission medium 20 shown as a line with an arrow head, and a receiver (RX) 30. The transmitter 10 transmits a signal into the transmission medium 20 that leads to the receiver 30. The transmitter 10 may be transmitting a digital data stream or an analog signal.
Referring to FIG. 1b, an insertion point 40 along the transmission medium 20 is illustrated. The transmission medium 20 between the transmitter 10 and the point 40 has a portion 20a and a portion 20b. The choice of the insertion point 40 depends upon testing requirements. For example, when the transmitter 10 is tested, the point 40 is located proximate to the transmitter 10. When a signal propagated through the link 20 is to be characterized, the insertion point 40 is located proximate to the receiver 30.
Turning to FIG. 1c, a diagnostic apparatus 100 is shown. The diagnostic apparatus 100 is connected at the insertion point 40. For an analog link the diagnostic apparatus 100 can be used to gauge the output of the transmitter 10, recording the content arriving from the transmitter 10, or some other feature arriving along the portion 20a of the transmission medium 20. Also for analog links, the diagnostic apparatus 100 can be used to inject noise or a test signal into the portion 20b of the transmission medium 20, to be carried toward the receiver 30.
For a digital link, similar diagnostic activity could be carried out. The diagnostic apparatus 100 can monitor the output power, jitter, and noise of a signal transmitted by the transmitter 10. Alternatively or simultaneously, the diagnostic apparatus 100 can look for deficiencies in the transmission medium 20a and monitor the test signal. The diagnostic apparatus 100 can modify the test signal by delaying it, by jittering it, by inserting errors into it, by replacing the digital content, by attenuating, by amplifying the signal, or by “stressing” the signal in some other manner. The diagnostic apparatus 100 is normally expected to report aspects of the transmitter 10 and of the transmission medium 20. The results of “stressing” activity are typically reported at the receiver 30 or at some later point in the system being diagnosed.
To provide diagnostics to test or verify all these requirements for all of the links simultaneously is resource intensive and expensive. Consequently, there is a need to reduce the costs by sharing one set of diagnostic equipment among several active communication links. It is essential to be able to perform such characterization without disrupting operation of other communication links of the system being tested.